shared-qmi: fix warnings with -Wswitch-default

mm-shared-qmi.c: In function ‘set_current_capabilities_step’:
  mm-shared-qmi.c:601:5: error: switch missing default case [-Werror=switch-default]
    601 |     switch (ctx->step) {
        |     ^~~~~~
  mm-shared-qmi.c: In function ‘load_current_capabilities_step’:
  mm-shared-qmi.c:868:5: error: switch missing default case [-Werror=switch-default]
    868 |     switch (ctx->step) {
        |     ^~~~~~
  mm-shared-qmi.c: In function ‘setup_carrier_config_step’:
  mm-shared-qmi.c:2473:5: error: switch missing default case [-Werror=switch-default]
   2473 |     switch (ctx->step) {
        |     ^~~~~~

src/mm-shared-qmi.c

@@ -637,6 +637,9 @@ set_current_capabilities_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        g_assert_not_reached ();
     }
 }
 
@@ -898,6 +901,9 @@ load_current_capabilities_step (GTask *task)
         g_task_return_int (task, priv->current_capabilities);
         g_object_unref (task);
         return;
+
+    default:
+        g_assert_not_reached ();
     }
 }
 
@@ -2542,6 +2548,9 @@ setup_carrier_config_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         break;
+
+    default:
+        g_assert_not_reached ();
     }
 }
 
@@ -3034,6 +3043,9 @@ load_carrier_config_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         break;
+
+    default:
+        g_assert_not_reached ();
     }
 }